Processing, Preaging, and Aging of NiTi-20 at.% Hf High-Temperature Shape Memory Alloy from Laboratory to Industrial Scale

Abstract

High-temperature shape memory alloys (HTSMAs) are a growing area of research in active materials that exhibit recoverable transformation strain and shape memory responses. They show promise for applications such as actuators in the aerospace, automotive, and energy exploration industries. NiTiHf is a cost-effective HTSMA solution with increased transformation temperatures above 115 °C. In this study, processing and aging treatments were performed on an industrially produced, hot-extruded Ni50.3Ti29.7Hf20 (at%) HTSMA. Portions of the extruded material were hot-rolled, pre-aged for 12 h at 300 °C, aged for 3 h at temperatures ranging 450–750 °C, or a combination. The goal of the aging treatments is to establish coherency between the interface of the matrix and H-phase nano-precipitates in the HTSMAs. After processing, the samples were characterized by differential scanning calorimetry, Vickers hardness testing, scanning electron microscopy, and synchrotron radiation x-ray diffraction. Previous studies have shown that pre-aging is effective for controlled nucleation of H-phase precipitates in low Hf content NiTiHf high-temperature shape memory alloys, this study shows that pre-aging of high Hf containing NiTiHf high-temperature shape memory alloys plays a small contributing role on thermal or mechanical properties, but is essentially ineffective and unnecessary at the pre-aging conditions examined here. Additionally, it is shown that hot-rolling can be used to control the orientation of H-phase in NiTiHf HTSMAs.